Equine strangles is an ancient disease of horses that continues to be a major problem in the modern era. Strangles has many properties of a disease that could be controlled or even eradicated. The agent of strangles Streptococcus equi ssp. equi has a very narrow host range, infecting Equidae almost exclusively. The majority of infected horses generally recover with robust immunity and are resistant to re-infection. It is therefore conceivable that the stimulation of systemic and local immunity via vaccination could assist in the control of this disease. One major obstacle in the control or eradication of this disease is the lack of a safe and efficacious vaccine. The production of a safe and efficacious vaccine has been difficult. Currently available modified live vaccines lack safety and, to date, subunit vaccines have been ineffective in preventing experimental challenge. The goal of this work is the development of a gene deletion mutant vaccine which would be not only safe and efficacious but readily identifiable. To this end we investigated the currently available modified live vaccine strain for safety, efficacy and the molecular mechanism of attenuation. First we performed a simple safety and efficacy study in ponies using the PinnacleRTM IN vaccine. We found that while the vaccine was safe in ponies over a year of age with some background immunity, it caused clinical disease in immunologically naive, 6 month old ponies. Simultaneously we developed a method to detect the vaccine strain from clinical isolates from horses with abscesses using the BiologRTM Microphenotypic array, pulsed-field gel electrophoresis and sequencing for single nucleotide polymorphisms (SNPs). To understand the molecular mechanism of relative attenuation of this strain, we interrogated the genome of this strain using comparative genomic sequencing. Using this method we identified 61 SNPs in coding regions and 26 SNPs in non-coding regions. Within the 61 identified genes, 10 have the potential to influence the growth or virulence of this organism. These 10 genes will be the target of further studies. Next we developed a high throughput animal model to screen for attenuation of virulence gene deleted mutants. We showed that an intramuscular injection model using the zebrafish host was able to detect relative attenuation of gene deletion mutant strains. We confirmed these results using a comparable mammalian model: an intraperitoneal injection mouse model. Using these models we have developed a triple gene deletion mutant which will be tested in horses in future studies. During our experiments, we also noted that the zebrafish host developed a robust cellular inflammatory response to Streptococcus zooepidemicus that was not observed in infection with S. equi. Using whole genomic zebrafish expression arrays, we observed increased expression of cytokines and factors important for growth and differentiation of phagocytes in fish injected with S. zoo while acute phase proteins and antimicrobial lectins were up-regulated in fish injected with S. equi.